The present invention relates to improvements of nuclear fuel assemblies and more particularly to an method of using a channel covering a bundle of fuel elements and to an improved channel structure for the method.
In a nuclear reactor, fuel elements which contain nuclear fuel are provided with tubular claddings and grouped together at fixed distance from each other in a coolant flow channel as a fuel assembly. Sufficient fuel assemblies are enclosed in a reactor vessel to form a reactor core. The heat generated by the nuclear fuel is removed by passing a coolant inside the channel from the bottom to the top of the fuel elements. The channel is made of thin plate and usually has a square cross section and a tubular shape. Exemplary of the prior arts are U.S. Pat. Nos. 3,350,275 and 3,338,791, which disclose precise structure of the conventional fuel assemblies.
The reaction continues as long as sufficient fissionable material exists in the fuel elements, however, the life time of the channel is at most 50,000-60,000 hours. Usually, after the life time of the channel has passed, the channel is detached from the fuel assembly and a new channel is attached in place of the removed channel over the bundle of the fuel elements. The used (removed) channels are disposed of as radio active wastes.
This shortend life time of the channel is caused by the fact that, considerable difficulty has been found after 4-6 years use of the channel, that is, the channel deforms due to pressure difference between inside and outside of the channel. Under an usual operation, high pressured coolant flows inside the channel to cool the fuel elements and the channel has always outside force applied to deform by the pressure difference between inside and outside of the channel. Still more, the heat and the irradiation promote the deformation. The amount of the deformation is greater in the lower portion of the channel and smaller in the top and the middle of the channel.
One of the most serious problems associated with the deformation of the channel is that the passage defined between adjacent channels for the movement of the control rod becomes narrower, therefore it becomes impossible to control the operation of the reactor by the control rod.
Another problem associated with the deformation is that the coolant flowing inside the channel leaks at an opening produced by the deformation at the connecting point of the channel and the lower tie plate, therefore an effective cooling for the fuel element is not obtained.
Further problem associated with the deformation is increase of the radio active wastes due to the shorter lasting quality of the channel, which will be a great problem for the environmental pollution.